# [Numpy-discussion] matrix multiplication A x, x has some zeros

Bruce Southey bsouthey@gmail....
Fri Dec 14 08:33:08 CST 2007

```Hi,
How sparse are A, x and Ax?

You probably will not find any time or operation efficiency unless A
and x are relatively sparse. With the small size of n, any efficiency
will typically be small in any case.

As you indicate that this has to be repeated multiple times, perhaps
you should look at your overall algorithm for efficiencies.

Bruce

On Dec 14, 2007 3:44 AM, dmitrey <dmitrey.kroshko@scipy.org> wrote:
> I guess it doesn't matter, but typical n are 1...1000.
> However, I need to call the operation hundreds or thousands times (while
> running NLP solver ralg, so 4..5 * nIter times).
> Number of zeros can be 0...n-1
>
> Charles R Harris wrote:
> > How big is n?
> >
> > On Dec 14, 2007 1:24 AM, dmitrey <dmitrey.kroshko@scipy.org
> > <mailto:dmitrey.kroshko@scipy.org>> wrote:
> >
> >     Hi all,
> >     I have to get Ax, A  is n x n matrix, x is vector of length n.
> >     Some coords of x are zeros, so I want to economy time/cputime somehow
> >     w/o connecting sparse module from scipy.
> >     What's the easiest way to do so?
> >
> >     Thx, D.
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```